Chlorinated vinyl chloride resins, which are produced by additional chlorination of vinyl chloride resins, not only have advantages of vinyl chloride resins such as excellent weather resistance, flame retardancy and chemical resistance, but also have a heat distortion temperature higher by 20 to 40° C. than that of a vinyl chloride resin, and therefore are preferably used in applications that require heat resistance up to 100° C., such as heat-resistant pipes, heat-resistant fittings, heat-resistant valves, and heat-resistant plates.
Because of its high heat distortion temperature, however, a chlorinated vinyl chloride resin has to be heated and melted at a high temperature for molding into a heat-resistant pipe, a heat-resistant fitting, a heat-resistant valve, a heat-resistant plate, or the like. In this context, a chlorinated vinyl chloride resin with poor thermal stability (initial discoloration resistance, heat-resistant stability) etc. cannot be molded into a transparent product and therefore is disadvantageous.
Additional chlorination of a vinyl chloride resin is conventionally performed by preparing an aqueous suspension of a vinyl chloride resin in a hermetically sealable reaction vessel and subjecting the aqueous suspension to ultraviolet irradiation while injecting chlorine into the reaction vessel. Recently, for production of a chlorinated vinyl chloride resin with excellent thermal stability, a chlorination method that simply involves heating without ultraviolet irradiation (heat chlorination) was developed. In addition, for the purpose of reducing the reaction time, addition of hydrogen peroxide during heat chlorination has been suggested.
An example is “a method for producing a chlorinated vinyl chloride resin by suspending polyvinyl chloride in an aqueous medium in a hermetically sealable vessel, reducing the interior pressure of the vessel, and chlorinating the polyvinyl chloride at a temperature of 90 to 140° C. while injecting chlorine into the vessel, the method being characterized in that, in the course of the chlorination, addition of hydrogen peroxide at a rate of 5 to 50 ppm/hr relative to the polyvinyl chloride is initiated when the chlorine content of the resulting polyvinyl chloride reaches or exceeds 60% by weight” (see Patent Literature 1, for example). In this literature, Example 1 has the following description: “200 kg of deionized water and 56 kg of PVC having an average degree of polymerization of 600 were charged into a 300-liter glass-lined reactor, the resultant mixture was stirred for dispersion of PVC in water, and then the reactor was heated to raise the interior temperature to 70° C. Subsequently, the interior pressure of the reactor was reduced to remove oxygen (oxygen level: 100 ppm), chlorine (oxygen content: 50 ppm) was then injected so that the partial pressure of chlorine was kept at 0.4 MPa, and thus a heat chlorination reaction started. After this, the temperature was further raised to 100° C. When the chlorine content reached 61% by weight, addition of a 200-ppm hydrogen peroxide solution at 15 ppm/hr in terms of hydrogen peroxide relative to PVC was initiated. Under fixed conditions at 100° C. and 0.4 MPa of partial chlorine pressure, when the chlorine content reached 65% by weight, the supply of chlorine gas was terminated to stop the chlorination. Then, nitrogen gas was supplied for removal of unreacted chlorine, and the resulting CPVC slurry was neutralized with sodium hydroxide, washed with water, dehydrated and then dried to give CPVC as a powder.” As described above, Example 1 suggests a method in which chlorination is initiated at a low temperature, which is then raised to a predetermined temperature for further chlorination.
In the chlorination method described above, however, the temperature is not under control from the time when the chlorination starts with the injection of chlorine into the reactor until the time when the temperature has risen to the reaction temperature, but simply the temperature is allowed to rise as quickly as possible to the reaction temperature. For this reason, this method may produce a chlorinated vinyl chloride resin that is poor in thermal stability (initial discoloration resistance, heat-resistant stability) etc. and cannot be molded into a transparent product, and therefore is disadvantageous.